The angiogenic factor cysteine-rich angiogenic protein 61 (Cyr61), a member of the CCN protein family, is overexpressed in a biologically aggressive subset of breast carcinomas. Moreover, forced expression of Cyr61 in breast cells promotes tumor formation in nude mice. Our clinical evaluation of Cyr61 (n=189 paraffin embedded tumor specimens) revealed that 30% of invasive breast carcinomas overexpress Cyr61, with Cyr61 expression inversely correlating with estrogen-receptor expression and erbB-2 overexpression. These findings support the notion that Cyr61 is a novel prognostic marker for breast cancer progression. We have recently described that Cyr61 functions as a survival factor capable of decreasing breast cancer cell chemosensitivity to the microtubule-targeting agent Taxol, the drug of choice in the treatment of metastatic breast cancer. Additionally, we demonstrated that functional blockage of the Cyr61 integrin receptor alpha-v-beta3 with arginine-glycine-aspartate (RGD) peptidomimetics markedly decreases cell viability in MCF-7 cells engineered to overexpress Cyr61, whereas it had no effect on Cyr61-negative control cells. Furthermore, alpha-v-beta3 blockade caused Taxol sensitivity in MCF- 7/Cyr61 cells to return to that observed in Cyr61-negative control cells. We hypothesize that high levels of Cyr61 produced and secreted by breast cancer epithelial cells, bind to alpha-v-beta3 to stimulate growth in an autocrine manner. We hypothesize that Cyr61-dependent activation of alpha-v-beta3-driven cellular signaling actively regulates breast cancer cell survival, thus promoting a more aggressive and chemoresistant breast cancer phenotype. Consequently, we hypothesize that by interfering with the Cyr61-alpha-v-beta3 signaling we may concomitantly block metastatic progression and Taxol resistance in breast carcinomas. Our studies are designed not only to evaluate whether the interaction between Cyr61 and alpha-v-beta3 is necessary and/or sufficient to induce breast cancer tumorigenicity, metastasis and chemoresistance but also to determine whether Cyr61 plays a role in the etiology of breast cancer. Therefore, this systematic approach will clarify the prognostic and predictive value of Cyr61 in breast cancer disease. We have formulated the following aims: 1) To determine the impact of Cyr61 expression in breast cancer progression and to evaluate the significance and clinical contribution to the treatment decision, 2) To assess the contribution of Cyr61 in the initiation and/or promotion of breast cancer. We will determine how Cyr61 contributes to the preneoplastic transformation of non-committed breast epithelial cells (normal cells) and whether Cyr61 overexpression triggers the malignant development of pre-neoplastic lesions in the breast, 3) To evaluate whether the specific silencing of Cyr61 gene expression modulates breast cancer progression and sensitivity to chemotherapy-induced cell damage, 4) To determine the involvement of the Cyr61 receptor alpha-v-beta3 integrin in Cyr61-induced breast cancer progression and chemoresistance, and 5) To determine whether a functional interaction between Cyr61 and alpha-v-beta3 is necessary and/or sufficient to induce malignant transformation, metastatic progression and/or chemoresistance in breast epithelial cells. In summary, we propose a previously unrecognized mechanism of breast cancer etiology, aggressive progression and chemosensitivity, in which the different compartments of breast cancer do not develop resistance through heritable genetic mutations, rather, as a result of their exposure to pro-angiogenic survival/growth factors present in the tumor microenvironment (i.e., Cyr61), become metastatic as well as refractory to chemotherapy by activating autocrine/paracrine pro-survival loops (i.e., Cyr61/alpha-v-beta3). In addition, we will evaluate the relevance of a Cyr61- triggered "Cyr61/alpha-v-beta3 integrin loop" as a novel predictive marker for response to chemotherapy in breast cancer.